Prosthetic implant failure mechanisms are numerous. Among the most prevalent causes of failure are polyethylene wear, aseptic loosing, infection, and malalignment. Polyethylene wear comprises the largest single identifiable cause of implant failure today. Moreover, polyethylene wear can predispose implants to loosing as a result of increased loading of the reformed tissues. Also, as implant technology evolves, new and more complex modes of wear, damage and failure are being identified. As a consequence of these facts, there is a great need for rigorous implant life cycle testing in simulator machines that are capable of replicating the subtleties of human motion.
Simulator machines address the implant longevity problem by providing a non-human environment in which to evaluate new and existing prosthetic devices through accelerated life testing. These machines allow researchers to isolate and study design deficiencies, identify and correct materials problems, and ultimately to provide the physician and patient with longer life prosthetic systems. Simulator machines approximate human joint motion, and clearly, the closer the approximation the more reliable the results.
To date, simulator machines have at best provided only a very rough approximation of the complexity of human joint motion, such as knee motion. So-called displacement controlled machines rely on an a priori description of the kinematics of the relevant body part, making little or no allowance for variations in prosthetic design, and subject the implant device to these prescribed motions for the duration of the life cycle test. Other machines use a force control system that subjects the prosthetic device to an ensemble of forces and torques which represent those encountered in the body part (e.g. a knee) during physiologic motion. Once implanted in the patient, however, the prosthetic is supported and constrained by the soft tissues of the body. Hence, for improved accuracy, force controlled machines should in some way simulate the natural constraints of these soft tissue forces. Some simulator machines have attempted to provide such constraints with a complex system of mechanical springs. However, these springs have proven cumbersome to work with, and have only a limited capability of simulating the complex characteristics of the human body, such as the knee's soft tissue.